An electrocardiogram, or ECG, is an important aid in the diagnosis of heart condition. It is a graphic tracing of the electric current generated by the heart muscle during a heartbeat. The tracing is recorded with an electrocardiograph (historically a relatively simple string galvanometer), and it provides information on the condition and performance of the heart. Electrocardiograms are made by applying electrodes to various parts of the body to lead off the miniscule heart current to the recording instrument. The four extremities and the chest wall have become standard sites for applying the electrodes. After the electrodes are in place and conductive contacts are assured through the application of a salt paste, the instrument may be calibrated by the application of a standard voltage from a source outside the body. Standardizing electrocardiograms makes it possible to compare them as taken from person to person and from time to time from the same person.
The normal electrocardiogram shows typical upward and downward deflections that reflect the alternate contraction of the atria (the two upper chambers) and of the ventricles (the two lower chambers) of the heart. The first upward deflection, P, is due to atrial contraction or depolarization of the atria and is known as the atrial complex. The other deflections, Q, R, S and T, are all due to the action of the ventricles and are known as the ventricle complexes. The QRS wave (sometimes referred to as an R-wave, the predominant wave of the group), represents the depolarization of the ventricles, while the T-wave represents the repolarization of the ventricles. (It is noted that the atria also are repolarized, but this atrial repolarization occurs at approximately the same time as the depolarization of the ventricles; and any electrical signal generated by atrial repolarization is generally minute and is masked out by the much larger QRS-wave on the electrocardiogram.)
Any deviation from the norm in a particular electrocardiogram is indicative of a possible heart disorder. Information that can be obtained from an electrocardiogram includes whether the heart is enlarged and where the enlargement occurs, whether the heart action is irregular and where the irregularity originates, whether a coronary vessel is occluded and where the occlusion is located, and whether a slow rate is physiological or caused by heart block. The presence of high blood pressure, thyroid disease, and certain types of malnutrition may also be revealed by an electrocardiogram.
The technology of cardiac pacemakers has developed to a high level of sophistication of system performance. The current generation of cardiac pacemakers incorporates microprocessors and related circuitry to sense and stimulate heart activity under a variety of physiological conditions. These pacemakers may be programmed to control the heart in correcting or compensating for various heart abnormalities which may be encountered in individual patients.
The interpretation of electrocardiograms is not a simple task. It generally requires specialized training and even then opinions may differ regarding the interpretation of particular configurations of heart signal waveforms and concomitant symptom. In some cases certain symptoms may be overlooked as, for example, when the corresponding signal waveform aberration occurs only rarely on an intermittent basis.
The use of a pacemaker in stimulating heart activity generally complicates the task of interpreting corresponding electrocardiograms. The stimulating pulses tend to induce a certain artificiality to the heart signal waveform with a resultant electrocardiogram that is even more difficult to interpret if the pacing pulses are sometimes present and then occasionally inhibited because of intermittent normal heart activity.
In order to efficiently perform its function as a pump, the heart must maintain a natural AV synchrony. The term "AV synchrony" relates to the sequential timing relationship that exists between the contractions of the atria and the ventricles. The P-QRS-T cycle of waves represents the natural AV synchrony of the heart. These waves, including the time relationships that exist therebetween, are carefully studied and monitored through conventional ECG techniques whenever the operation of the heart is being examined.
An electrocardiogram is the primary tool for checking on the functioning of a pacemaker after it has been implanted. The record can be made in real time and, as noted, commonly consists of a tracking on a strip chart recorder. The strip chart usually consists of graph paper on which the smallest division is one millimeter, with every fifth line being accented to outline larger squares which are five millimeters on a side. The continuous strip of graph paper is moved beneath the recording pen at such a rate that each millimeter square corresponds to 40 milliseconds (ms), and each larger square corresponds to a time period of 200 ms.
It is common practice for a cardiologist to analyze an ECG from a patient using a pair of calipers or dividers. Using dividers, a cardiologist can measure a time interval between any two events occurring on the ECG trace. For example, to determine the pacing rate of a patient's single-chamber pacemaker, the cardiologist measures the distance between any two consecutive pacing stimuli. To accomplish this, the points of the divider are placed on the ECG trace between two consecutive pacing stimuli and then, with the divider spacing set, the pointers are placed on the ECG strip starting at some fixed reference line to measure the pacing period. Since there are 60,000 ms in one minute, dividing the pacing period in ms into 60,000 yields the rate in beats per minute. Also, the dividers set to the pacing period may be placed elsewhere on the ECG trace to compare subsequent intervals of similar pacing stimuli.
In determining the pacing rate from an ECG trace for a dual-chamber cardiac pacemaker, measurement of two intervals is required, namely the time from an atrial stimulus to a ventricular stimulus (AV interval) plus the time from the ventricular stimulus to the next atrial stimulus (VA' or atrial escape interval). Using a standard pair of dividers, a cardiologist must reset the dividers between measuring the AV and VA intervals and must again and again reset the dividers when comparing such intervals with similar subsequent intervals. Such operations are time consuming and subject to inaccuracies.
One particular caliper-type device, which has been marketed, is known in the prior art as a Trivider caliper. This caliper comprises a divider having three legs which allow simultaneous measurement of time intervals between any three events occurring on an ECG trace (e.g., AV and VA) without resetting or adjusting the caliper.
It would be a great boon to the art of analyzing pacemaker ECG traces if there were a device that allowed the measurement of more than two intervals at one time. If such a device were lightweight, compact, and lacked the sharp points associated with calipers, it would be easier to carry in one's pocket and would be safer and more convenient to use.